1. Field of the Invention
The present invention relates to an electronically controlled camera in which a lens is electronically driven to move within a movable range along an optical axis thereof.
2. Description of the Related Art
Conventionally, electronically controlled cameras have a photographing lens that is driven by a motor and moves within a predetermined movable range. In such a camera, for example, a zoom lens can move within a zooming range, which is defined as a range between a wide end and a tele end. The zoom lens is driven to move between the wide and tele extremities to change the focal length of the photographing lens.
In such a type of camera, the lens is positioned in the movable area when a main switch, or a power switch of the camera, is turned ON and the camera is in the condition where photographing can be performed. The lens is positioned at a retracted, or accommodated position (housed position) where the lens is retracted and barrier blades cover the lens when the power switch of the camera is turned OFF.
In the conventional camera as described above, when the power switch of the camera is operated and the operating condition of the camera changes from an OFF condition to an ON condition, the photographing lens, which was positioned at the housed position is extended and located within the movable range (i.e., a zooming range).
Within the zooming range, the lens is moved towards the tele extremity (tele end) or the wide extremity (wide end) in accordance with operation of a zoom lever or the like. When a user stops operating the zoom lever, the movement of the lens stops. In this situation, when the power switch is turned OFF, the lens positioned in the zooming range is moved to the housed position as described above.
In the electronically controlled camera, especially one which controls the motor to move the lens, the position of the lens should be detected to determine the focal length of the lens. For this purpose, a code plate is generally employed.
An example of the conventional code plate is a plate on which a certain pattern forming a plurality of codes corresponding to the lens position is provided. Each code consists of a plurality of conductive and non-conductive elements. Further, a detector for detecting a code is provided. The detector may have a plurality of brushes which simultaneously contact different portions of the code plate, and detect the conductivity of respective portions. That is, each brush detects the conductivity of the portion where it contacts. Combinations of the conductive and non-conductive portions represent the code which indicates the position of the lens. As the lens moves, relative movement between the code plate and the detector occurs. Based on the detected code, i.e., the detected combination of the conductive and non-conductive portions, the position of the lens, which corresponds to the focal length of the lens, is detected.
In a code detecting system as described above, if the code is not detected correctly, a wrong lens position may be detected as the lens position. In such a case, the lens may not be driven correctly, and may be driven to be positioned at a wrong place. Therefore, in the camera employing the code plate and the detector of the code, the code should be detected correctly, to ensure that the correct lens position is detected.
In order to obtain the accurate position of the lens, various methods have been suggested.
For example, a conventional lens position detection system is provided with a code plate having conductive and non-conductive portions forming various codes, and a plurality of brush members which contact the code plate to detect a code as described above. One of the brushes and the code plate move in accordance with the movement of the lens, and the other does not move (i.e., the positional relationship is fixed with respect to the body of the camera). Depending on the position of the lens, the brush contacts a pattern of the conductive and non-conductive portions indicative of the lens position. In other words, each code consists of a plurality of conductive and non-conductive portions, and the brushes detects the combination of the conductive and non-conductive portions. Therefore, each code represents a binary information indicating a plurality of ON and OFF states.
However, with the conventional lens position detection device as described above, in order to identify a number of lens positions, the large number of brushes and corresponding conductive and non-conductive positions should be provided since the plurality of positions are to be indicated with a binary information (i.e., a combination of conductive and non-conductive portions).
In the detection device constructed as above, it is highly probable that one of the brushes may not contact the code plate, i.e., the brush is lifted from the code plate. Under such a situation, the detected code is different from the actual code since the bit corresponding to the lifted brush always indicates the OFF state, and the wrong lens position is detected.
Further, in the conventional lens position detecting device constructed as above, one of the brushes should be grounded regardless of the lens position, and the other brushes contact the conductive and non-conductive portions in order to determine the conductivity of each of the portions contacting the brushes. In order to achieve this, a signal from each brush is input into a control unit, e.g., a CPU (central processing unit), and accordingly, relatively complicated wiring is required.
A further problem is that when the lens is driven and extended from the housed position to the wide end, the control unit of the camera continues driving the lens until a code indicating the wide end is detected. In response to the detection of the code indicating the wide end, the control unit stops driving the lens. If an error occurs, however, and the code indicating the wide end is not detected even through the lens reaches the wide end, the control unit continues driving the lens past the wide end. In such a case, the lens may move beyond the originally designed movable range, which is defined by, for example, the wide and the tele extremities (ends), and the lens may contact a mechanical stopper or the like which may be provided at the end of the movable range. Further, in a worse case, a screw part of the lens driving mechanism may be driven excessively or may be fastened at a part beyond the movable range.
A similar problem may occur when the lens is moved to the wide end. When the zoom lens is moved towards the wide end while the lens is located within the movable range, in response to the detection of the code indicating the wide end, the control unit stops driving the lens. In other words, the lens is prevented from moving towards the accommodated position (housed position), beyond the wide end when the camera operates. However, if the control unit fails to detect the wide end while the zoom lever is being operated and the lens moved towards the wide end, the control unit keeps driving the lens. In such a situation, fastening of the driving mechanism (i.e., the screw mechanism) at the housed position may occur, and/or an excessive load may be placed on the driving motor.
A similar problem may also occur when the main switch of the camera is turned OFF and the lens is to be retracted into the housed position. Conventionally, when the lens is moved to the housed position, it is detected whether the lens has reached the wide end. When the lens has reached the wide end, a predetermined operation such as a preparation operation for lens accommodation is performed. Thereafter, the lens is further moved to the housed position. In such a camera, if the detection of the wide end fails, the lens continues to be driven since the wide end code is not detected, which may cause damage to the motor and/or mechanical fastening of the lens driving mechanism.
Furthermore, with the above type of camera, when the lens is moving the code changes and the detection device detects various codes one-by-one. If the movement of the lens is prevented while attempting to move the lens, the codes stop changing even though the driving motor continues performing the lens driving operation. If the driving mechanism continues the lens movement operation under such a situation, the driving mechanism may be damaged or break. To avoid such a problem, in a camera in which the lens position is detected with use of the code plate and the code detection device, it is determined whether the code changes within a predetermined period of time. If the code does not change within the predetermined period, it is judged that an error has occurred, and the control unit stops the lens driving operation.
In the above-described situation, the load applied to the lens driving motor may be less if the predetermined period is set shorter. It is desirable to set the predetermined period as short as possible since it is desirable that the motor is stopped if the movement of the lens is prevented.
Generally, the lens is prohibited from stopping within an area between the accommodated position and the wide end. In a camera in which the lens position is detected using the code plate, there is no need to detect the lens position within the above-described area where the lens is prohibited from stopping, and therefore, there is no need to provide a code within the area.
Accordingly, a code is not formed in the area where the lens is prohibited from stopping. In other words, since the code does not change while the lens is moving the area between the accommodated position and the wide end, it takes a relatively long time for code change to occur. The code change occurs only after the lens has reached the wide end. Thus, the above-described predetermined period for detection of the code change should be set to a relatively long period. In order to wait the code change when the lens moves between the accommodated position and the wide end, the above-described predetermined period should be set to a relatively long period of time contrary to the desired short period.
The period of time set for detection of the code change therefore should be set to a relatively long value. Accordingly, if the lens movement is prevented while the lens driving motor is driven until the larger predetermined period has elapsed, and the motor is continuously driven during this period, the burden on the motor is increased.
Further, there has conventionally been known a zoom lens camera in which zoom lens is moved when a user operates an operable member, such as a zoom lever so that the focal length of the photographing lens can be changed. In such a camera, generally two types of movement control of the zoom lens are known.
A first is that when the user stops operating the operable member, the lens stops moving immediately. Then, when the user pushes a shutter button of the camera, the zoom lens remains where it is located. In other words, the zoom lens is capable of stopping anywhere within its movable range.
Another type is such that the zoom lens stops at only one of a plurality of predetermined positions when the user stops operating the operable member. In other words, in the latter type, the zoom lens moves in a stepwise fashion.
Further, the amount of movement of a focusing lens and an aperture value, which is used for exposure control, vary as function of the focal length. Accordingly, the latter type has an advantage in that the processing performed by a controller of the camera is less since the amount of information to be used in the exposure control. Additionally, exposure calculation parameters and the like are less than in the former type. Therefore, the latter type of the zoom lens and movement control thereof are widely employed in cameras, especially in compact cameras.
In the latter type, however, since the zoom lens stops at a position within the movable range of the lens, the lens position may deviate from the position where the lens is expected to stay after the user stops operating the operable member to stop the lens, and before the shutter button is depressed. When the lens deviates from the predetermined position, the focal length, which the exposure and focusing controls are performed based upon, changes and therefore the exposure and focusing controls become inaccurate.
Furthermore, the conventional camera may employ a "relative" code system, which has an advantage such that the number of lens positions can be identified using a fewer number of codes. In the relative code pattern system, the actual lens position is determined with reference to the detected relative code and the reference code.
In the camera employing the relative code system, when the lens is forcibly moved, the control unit may determine the wrong position as the lens position. If the lens movement is performed with reference to incorrectly determined position, the resultant position is also in correct.